This invention relates to compositions for the preservation of timber.
It is known to use pesticides of the pyrethroid class to protect timber from attack by insects and other pests (see Japanese Patent Application No. 52-66603; 1975; Sumitomo). However, pyrethroids are relatively expensive pesticides and this can be disadvantageous when a high volume, low cost material such as wood is being treated. Furthermore, pyrethroids are known to be short-lived when exposed to soil. The most commonly used timber-protecting pesticides, in many cases, are thus still relatively toxic but cheap compounds such as lindane (gamma-hexachlorocyclohexane).
It has also been known for many years to use creosote to protect timber. Creosote has the advantage of being cheap, but wood treatment with creosote is unpleasant to handle, being oily and smelly. In addition, the creosote has to be applied to the timber in such quantities that it may later bleed out of the timber again and cause a fire hazard. One solution which has been adopted is to pretreat the timber with CCA (a mixture of copper, chromium and arsenic compounds) before treatment with creosote. However, CCA is fairly toxic, and a two-stage treatment process adds to the cost of the treated timber. It is not possible to mix CCA with creosote to give a single step process, as the mixture is not stable. Indeed, the problem of instability is encountered with many such mixtures. UK Patent Application No. 2 038 636 (Sumitomo) disclose the use of the insecticides fenvalerate and fenpropathrin to protect timber. It is stated that these compounds may be combined with a fungicide to obtain additionally a fungicidal effect, and creosote is mentioned, in a long list of other fungicides, as being suitable for this purpose. It has now been found that many of these fungicides are unsuitable, as the insecticide is degraded. It is stated in the Sumitomo specification that "when contact between wood and soil is unavoidable . . . an aqueous dilute solution containing the present composition . . . may be mixed with or injected into soil around the wood . . . "
It has now suprisingly been found that mixtures of certain halogenated pyrethroid insecticides and creosote are relatively stable and have a relatively prolonged protective action even when the timber is exposed to soil, without any need to treat the soil as well.
In a report dated August 1980 and sent to a limited number of organisations, the Penarth Research Centre (of Winchester, Hants, UK) stated that "tar oil and organic solvent preservatives intended to give protection against marine borers should always contain organochlorine insecticides to give improved protection against Crustacean borers such as Gribble. It has now been shown that pyrethroids have similar advantages. Whilst these insecticides are expensive, low retentions can achieve much more efficient control than even exceptionally high retentions of normal fungicidal preservative components". There was no suggestion that timber for burial in the soil could also be treated advantageously in the manner, and it was not stated or shown that pyrethroids would be stable in tar oils or the like.